No lawyer here, but it seems to be a similar issue to the pretty well-developed law of Trespass. It's going to end up revolving around whether someone is "knowingly" usurping a connection, whether a reasonable person can assume that a particular connection is in the public domain, and whether the use is incidental or wanton. Either that or we become a police state.
The other aspect is that it's not clear who's potentially committing the crime. If I allow my neighbor to run a cable onto my property and tap a connection it's mainly the owner of the connection that's afoul of the current laws. Wifi is even murkier -- more like setting up a big tv visible from the neighbor's house or the street. Anyone in view with a universal remote bought at Wal-Mart can then watch whatever they want.
Some are old enough to remember the days when AT&T had a monopoly on phone service. It was against the law (felony, I think!) to hook up additional devices, e.g. answering machines or extensions, in your home without reporting them to Ma Bell and paying an extra fee. I remember my dad, MHRIP, conspiratorially wiring extensions and instructing the family what to do if the Phone Company called or paid a visit. Technically, you see, the additional devices had a non-zero "ringer equivalent" and thus "used" some additional Phone Company resources.
IMO, there's probably more to the St. Petersburg case then was reported so far that lends itself to prosecution.
In almost all cases chaser use of a wifi connection is incidental and far under any possible Federal radar. They're also generally business or public connections, rather than residential, and the law of Trespass works differently for them. For example, if I walk into a Burger King, set up my tripod and start to take pictures, the manager can tell me to stop or leave. If I don't, then it's Trespass. On the other hand, if I walk onto the patio of a stranger's house and do the very same thing, it's Trespass then-and-there because I didn't have any business to be there, and a reasonable person knows that.